Sensing Spatially Structured Non-Paraxial Light Fields

What we can see with our own eyes or observe using standard optical imaging systems is limited to a small fraction of the information that the detected light actually carries. Two-dimensional (2D), flat images, such as a photo, only reveal the intensity and color of the light coming to us from an optical scene. However, light contains a wealth of information on the three-dimensional (3D) position, angle of incidence, spectral context, coherence, amplitude, phase, polarization, optical angular momenta, among others. In fact, if light interacts with media, nature will give us structured light [1] spatially varying in the named properties depending on the interaction. Different techniques nowadays allow for the analysis of some of these properties, e.g. digital holographic phase contrast microscopy, but they require complex experimental systems with bulky non-versatile optical components and/or data post-processing. Especially, 3D non-paraxial structured light could not be made available so far, thus, crucial information about the nature of interacting media stays hidden. The difficulty, in this case, lays in capturing not only the nanoscale complexity but also the 3D polarization nature of light, i.e., significant contributions of longitudinal in addition to transverse electric field components. The ability to access these properties could have a transformative impact on the development of imaging systems for today’s world, as in robotics, surveillance, augmented/virtual reality, (bio)medicine, or optical fields as nanophotonics.<br/>To extract the full electric field information of non-paraxial structured light to advance today’s imaging technologies, recent advances in nanophotonics can provide promising innovative tools and methods to perform this task. Following this path, we create a molecular nanosensor which is able to reveal the typically invisible non-paraxial field properties of light by a single camera shot [2]. Our sensor is formed by a functional monolayer of self-assembled fluorescent sulforhodamine B silane produced on a silica glass cover slip. The arranged dipoles in the monolayer interact with a spatially varying non-paraxial electric field, giving a characteristic response which is sensitive to the amplitude, phase, and 3D polarization of light.<br/>To demonstrate the capability of our approach, we introduce non-paraxial field customization by tightly focusing paraxial structured laser light fields [2,3]. We tailor these paraxial input fields by an amplitude, phase, and polarization modulation system [4] with a phase-only spatial light modulator as key component. This customization approach allows providing exemplary non-paraxial fields for our investigation on demand. We numerically as well as experimentally demonstrate the resulting, characteristic response of our monolayer sensor to these fields, successfully visualizing the non-paraxial field properties.<br/>In an inverse approach, taking advantage of on-demand light fields, we can apply customized non-paraxial fields for selective dipole excitation. By adapting the input field properties, we are able to adjust the dominating direction of electric field oscillation and match it to the orientation of the selected dipole. Thus, our approach holds the potential for the in-depth analysis of, for instance, plasmonic nanostructures, 2D materials, or single quantum emitters.<br/>[1] H. Rubinsztein-Dunlop et al. J. Opt. 19 (2017), 013001; E. Otte, <i>Structured Singular Light Fields</i>, Springer (2021).<br/>[2] E. Otte et al., Nat. Commun. 10.4308 (2019).<br/>[3] E. Otte et al., Opt. Express 25 (2017), 20194.<br/>[4] C. Alpmann et al., Sci. Rep. 7.8076 (2017).